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Trou au Natron is a volcanic caldera in the Tibesti Massif in northern Chad. The volcano is extinct, and it is unknown when it last erupted. Trou au Natron is located just south-east of Toussidé, the westernmost volcano of the Tibesti Mountains. The caldera has an irregular diameter of approximately 6 to 8 kilometres (4 to 5 miles) and is up to 1,000 metres (3,300 ft) deep. Because of its irregular shape, it has been theorized that the caldera was formed as a result of multiple massive explosions, each of which deepened the enormous pit. Its exact period of formation is unconfirmed, although a Pleistocene formation has been suggested. Much of the surface of the caldera is lined with a white crust of carbonate salts such as sodium carbonate and natrolite, known as natron, leading to the caldera's name, literally 'hole of natron' in French. This crust is sometimes known as the Tibesti Soda Lake. Both the slopes and the floor of the caldera contain thick layers of fossilized aquatic gastropods and diatoms, indicating that it was once home to a deep lake. This satellite image of Trou au Natron was taken in 2008 from the International Space Station, at an altitude of around 352 kilometres (219 miles). The white crust can be seen at the bottom of the caldera.Photograph credit: NASA

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